The present invention relates to signal monitoring in digital communication systems and in particular monitoring of a signal to noise ratio (SNR) in such systems.
In digital communication systems it is desirable that data is received free from errors. However, errors will arise due to noise inherent to the transmission system. One measure of the amount of noise in a received signal is a xe2x80x9csignal to noise ratioxe2x80x9d or SNR. In particular, the SNR level referred to herein is the ratio of signal peak amplitude to the standard deviation of the noise. This is one representation of SNR among many. The SNR, thus defined, is usually represented by the letter xe2x80x9cQxe2x80x9d. For instance, an SNR of 2 indicates that the amplitude of the received data signal is twice the RMS level of the noise. If a received signal is free of noise, the SNR is infinite.
Systems are designed to minimise noise on the transmission medium and to improve the system tolerance to noise. The system tolerance to noise is often quantified as a bit error rate (BER) at a particular SNR. Under certain conditions, such as component failure or electromagnetic interference, the SNR will be reduced such that the BER exceeds acceptable levels. By measuring the SNR on a data stream, it is possible to estimate the bit error rate (BER) caused by the noise and to know when the quality of a connection cannot support a required BER. A capability to monitor the SNR of a transmission system, then, is applicable to performance monitoring, fault detection, fault isolation, system test, system setup and other common telecommunication activities and functions.
In U.S. Pat. No. 5,796,479 issued Aug. 18, 1998 to Derickson et al., SNR is one of three qualities of a received optical signal which are monitored. A spectrometer spatially separates signals from wavelength division multiplexed (WDM) channels according to wavelength. The separated signals are incident on an array of split-detectors that conforms to the spatial separation of the signals provided by the spectrometer. While the split-detectors are positioned to receive a signal from each WDM channel, a noise detector is positioned between adjacent split-detectors to measure noise. A common mode output from two halves of each split-detector indicates the power in a WDM channel. The ratio of the common mode signal to the noise detector signal is used to monitor the SNR of each WDM channel. By virtue of measuring the noise outside the band of the channel of interest, the contribution to the overall noise power of in-band noise sources such as inter-symbol interference (ISI) is not taken into account. As well, noise introduced by such receiver components as a photodetector and preamplifier is not monitored.
The present invention discloses a method and apparatus for monitoring the signal to noise ratio (SNR) of a recovered data signal in a communication system. A signal representative of the difference between the input and output of a limiting amplifier in a receiver is assumed to be noise. A measure of the amplitude of the output signal is divided by a measure of the energy of the noise signal to result in an indication of the SNR of the input data signal.
In accordance with an aspect of the present invention there is provided a method of determining an indicator of signal to noise ratio in a noisy data signal including receiving the noisy data signal and limiting the noisy data signal to obtain a regenerated data signal. Subsequently, the method includes generating a noise signal by subtracting the regenerated data signal from the noisy data signal and further determining a magnitude of energy of the noise signal and an amplitude of the regenerated data signal. The method concludes by determining the indicator from the amplitude of the regenerated data signal and the magnitude of energy of the noise signal.
In accordance with a further aspect of the present invention there is provided a monitor for determining an indicator of a signal to noise ratio of a noisy data signal, based on input comprising the noisy data signal and a regenerated data signal, the monitor including a difference circuit for determining a noise signal comprising a difference between the noisy data signal and the regenerated data signal, an energy detector for determining a magnitude of energy of a signal output from the difference circuit, an amplitude detector for determining an amplitude of the regenerated data signal and a ratio circuit for determining an indicator from output of the energy detector and output of the amplitude detector.
In accordance with a further aspect of the present invention there is provided a monitor for determining an indicator of a signal to noise ratio of a noisy data signal, based on input comprising the noisy data signal and a regenerated data signal, the monitor including a first conversion means for converting the noisy data signal from analog to digital, a second conversion means for converting the regenerated data signal from analog to digital and a processor operable to receive a digital representation of the noisy data signal from the first conversion means, receive a digital representation of the regenerated data signal from the second conversion means, generate a noise signal by subtracting the digital representation of the regenerated data signal from the digital representation of the noisy data signal, determine a magnitude of energy of the noise signal, determine an .z amplitude of the regenerated data signal and determine the indicator from the amplitude of the regenerated data signal and the magnitude of energy of the noise signal.
In accordance with a further aspect of the present invention there is provided a receiver in an optical transmission system including a photodetector and preamplifier for converting a received optical signal to a noisy data signal, a limiting amplifier for limiting the noisy data signal to obtain a regenerated data signal and a clock and data recovery circuit for obtaining a retimed data signal and a recovered clock signal from the regenerated data signal. The receiver further includes a signal to noise ratio monitoring circuit operable to receive the noisy data signal and the regenerated data signal, generate a noise signal by subtracting the regenerated data signal from the noisy data signal, determine a magnitude of energy of the noise signal and an amplitude of the regenerated data signal and determine an indicator of a signal to noise ratio of the noisy data signal from the amplitude of the regenerated data signal and the magnitude of energy of the noise signal.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.